Single Board Computers (SBCs)
Single board computers (SBC) are always used in conjunction with a compatible backplane. Before you select any components for inclusion in your products, we always recommended that you first identify the market for your product, and also determine your market objectives. Once you have determined what your products need to be, and who will use them, you can proceed to select a backplane and SBC combination that accomplishes those objectives.
The primary benefit of using a backplane and SBC is the ease of service. Like systems that use an integrated main board, it is possible to troubleshoot modular SBC components including processor, memory, and flash storage. The biggest advantage of a single board computer is the ability to replace the entire processor card without removing any other boards or network cabling. If there is a failure, and you don't want to spend a lot of downtime troubleshooting components, an SBC makes it possible to easily swap out the entire processor card including all of its serviceable components. In contrast, the replacement of a main board requires the entire system to be disassembled, and then reassembled using the new main board.
Another advantage of using a passive backplane is the ability to upgrade the processor over time. As your end user's network demands grow over time, the processing capacity of an original product design may be outgrown, and a processor upgrade may be needed. When you use a passive backplane and SBC, it is a simple matter of removing the old processor card and installing the new one, and then copying configuration files, if needed.
SBC Form Factor
SBCs come in many different shapes and sizes. Some cards are designed to meet the standards for industrial PCI form factor (PICMG 1.0 and 1.2), while others are built to the CompactPCI form factor (PICMG 2.0). Some SBCs support a 32-bit bus, some support a 64-bit bus, and some provide multiple buses, including support for switch fabric. Some industrial PCI SBCs are full length, while others are half length. And some SBCs are designed to work with many different backplanes, while others will only work with a limited few.
Industrial PCI vs. CompactPCI
There are many variables that must be weighed when you select an SBC. The most important decision you will make is whether you are building products using standard industrial PCI or CompactPCI hardware. To simplify this process, the table below provides a list of selection criteria and the form factor that offers each capability.
| Industrial PCI vs. CompactPCI |
| | PCI | CPCI |
| Lowest Cost | Y | N |
| Smallest Footprint | Y | N |
| Hot-Swap Cards | N | Y |
| NEBS Compliance | N | Y |
| Extended Temperature | N | Y |
| Switch Fabric | N | Y |
| Telephony Bus | Y | Y |
| Dual PCI/CPCI Bus | Y | Y |
| PMC Card Support | Y | Y |
| Dual Processors | Y | Y |
Industrial PCI SBCs (PICMG 1.0 and 1.2)
Industrial PCI SBCs are generally designed to comply with the PICMG 1.0 or 1.2 standards. PICMG 1.0 is the first PICMG standard to provide specifications for industrial computers using the PCI bus. PICMG 1.0 compliant SBCs generally provide support for one 32-bit PCI bus and one ISA bus.
Variations on this standard include support for a 64-bit PCI bus, and a half-length format called PISA or PCISA which combines PCI and ISA bus connectors on a single card edge connector. Although PICMG 1.0 specifies support for the legacy ISA bus, this bus can be "hidden" in systems that do not need ISA by choosing a backplane that does not provide any ISA slots.
ISA cards have been discontinued by most manufacturers, which has led to the development of the PICMG 1.2 standard that supports dual PCI buses. The PICMG 1.2 standard (a.k.a. ePCI-X) provides a specification for dual bus systems that includes two PCI-X bus connectors on the SBC for connection to dual PCI buses on the backplane.
Full Length vs. Half Length
When it comes to industrial PCI systems, many more manufacturers produce full-length SBCs than half-length cards. This usually makes half-length SBCs cost more than full-length SBCs given a comparable class of processor. As a general rule, full-length cards are used in systems that have a relatively large footprint, and half-length cards are used in systems that require the smallest foorprint available in a passive backplane system.
Some hardware features that are available in a full-length card may not be available in a half-length card. For example, dual processor cards are always full length because half-length cards do not have sufficient space to put two processors. Obviously, it is impossible to build a half-length SBC when the circuit board does not provide enough space to mount the required components.
Processors
SBCs are available with a wide range of processor options. Although it is still possible to find SBCs that support 386 and 486 class processors, ImageStream only offers support for Pentium class systems and faster.
Many product applications can be handled by SBCs that support standard Pentium III or Pentium IV processors. For the most demanding applications, it may be necessary to specify an SBC that supports dual Pentium III or dual XEON processors. ImageStream also offers SBCs that provide low-power VIA C3 processors. Some of these SBCs can be deployed without a CPU fan.
Bus Support
After processor performance, bus support is typically the next most important feature of an industrial computer system. 32-bit PCI systems are straightforward because they have been around for many years, and they are widely supported by all industrial PC manufacturers. 64-bit systems come in many different configurations, including a variety of backplanes that offer many options with regard to the number and speed of the available expansion slots.
SBCs with a 64-bit PCI bus can potentially support expansion slot speeds of 33 Mhz, 66 Mhz, and 133 Mhz (PCI-X). When mated with an active 64-bit backplane, it is possible for the backplane to provide slots that support any or all of these clock speeds.
Conclusions
Selection of an SBC depends on many factors. For example, processor performance is a common factor that influences SBC selection. The interdependence of backplanes and SBCs makes it clear that the selected backplane must not only meet the application specifications, but it must also be compatible with an SBC that will also meet those specifications.
In the final analysis, the right SBC for a project cannot be specified until the backplane is specified, and the backplane cannot be specified until you know what cards will be used. It accomplishes nothing to select an SBC when it will not work with the backplane that you must inevitably adopt. Similarly, it does no good to select a backplane unless it will support the required cards. Once you have determined the number and type of expansion cards that your product will support, and once you have selected a backplane that will support those specific cards, then and only then can an SBC be chosen that will properly support the application.
There are so many variables associated with the selection of an SBC, you would need a computer to calculate the possible combinations. As you consider your options for SBCs and backplanes, it is a good idea to consult an ImageStream OEM product engineer who can help you analyze your product performance requirements in terms of processor, memory, and bus speed. With this information, the same ImageStream engineer can help you choose the best backplane and SBC for your product.
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